Harmful trace components such as mercury are present in exhaust gas resulting from the combustion of coal or heavy oil, and it is generally difficult to remove them in the existing exhaust gas treatment systems. It is believed that mercury exists in exhaust gas chiefly as metallic mercury (Hg) or mercury chloride (HgCl2). Since HgCl2 is easily absorbed into water, it can be removed in a desulfurizing absorption tower or the like. However, metallic mercury (Hg) has a very low solubility in water and cannot be absorbed in a desulfurizing absorption tower. Consequently, there is the possibility that metallic mercury vapor may be discharged from the stack.
For this reason, an activated carbon adsorption method, a sodium hypochlorite absorption method and the like have conventionally been employed as Hg removal techniques.
For the activated carbon adsorption method, a method in which activated carbon powder is blown into exhaust gas and recovered with a bag filter has already been put to practical use. However, this method is employed chiefly for the treatment of exhaust gas from refuse incineration, and its application to a large-volume gas such as exhaust gas from an electric power plant is not known.
For the sodium hypochlorite absorption method, there is a known method where an additive such as sodium hypochlorite is directly added, for example, to the cooling water of the cooling tower, the absorbing fluid of the desulfurizing absorption tower, or the feed water or circulating water of the wet dust collector. However, in all cases, an additive is added to a main unit in an exhaust gas treatment plant, and some additives involve the risk of interfering with its intrinsic function. For example, it is conceivable that the cooling water has a low pH and hence requires a large amount of an oxidizing agent, forming peroxides in the cooling tower, and oxidizing sulfurous acid in the wet dust collector to cause an increase in acidity. Moreover, this method has been adapted chiefly to the treatment of exhaust gas from refuse incineration, and is not suitable for the treatment of a large-volume gas such as exhaust gas from an electric power plant.
Meanwhile, metallic mercury is hardly soluble in water and hence passes through the desulfurizer, as described above. If metallic mercury can be made soluble in water, it may be removed in the desulfurizer. Accordingly, it is conceivable that, if metallic mercury can be converted into water-soluble mercury chloride on the catalyst of the denitrator, the mercury chloride may be removed in the desulfurizer installed downstream thereof. That is, an exhaust gas treatment method in which a chlorinating agent (e.g., hydrogen chloride) for converting metallic mercury into mercury chloride is injected on the upstream side of the denitrator is believed to be effective.
However, the addition of an excessive amount of the chlorinating agent involves a problem in that it can cause corrosion of the flue and downstream units of the system and eventually shortens the life of the plant equipment. Moreover, if the chlorinating agent is simply injected at a constant feed rate, this will cause an increase in utility costs.
More specifically, after the denitrator, an air heater, a dust collector, a gas-gas heater (heat exchanger) and a desulfurizing absorption tower are usually installed in that order. Yet amongst those, the chlorinating agent exerts a marked corrosive or damaging effect on the heat exchanger used for cooling purposes. Another problem is that since the chlorinating agent is introduced into the desulfurizing absorption tower, the chlorine concentration in the absorbing fluid increases causing corrosion or damage of the metallic parts of the tower. Moreover, an increase in chlorine concentration within the desulfurizing absorption tower may cause a reduction in oxidation capability during desulfurization or a reduction in desulfurization capability itself, leading to a reduction in the overall performance of the system. Furthermore, an increase in chloride concentration may cause an increase in the foamability of the absorbing fluid, possibility raising the pressure loss within the absorption tower and causing an increase in operating power.